Module 6: Underground & Tunnel Surveying
1. Gyrotheodolites and Underground Alignment
Precise underground orientation is critical for tunnel construction and mining operations.
Gyrotheodolite Fundamentals:
Uses a gyroscope to find True North underground, independent of magnetic interference.
Provides azimuth reference when GPS is unavailable (e.g., deep tunnels).
Accuracy: ±10–20 arc-seconds for high-precision alignment.
Applications:
Tunnel Boring Machine (TBM) Guidance: Ensures tunnels stay on design alignment.
Shaft Plumbing: Transfers surface coordinates vertically into mines.
Military & Infrastructure: Used in subway systems and underground utilities.
Workflow:
Surface survey establishes control points.
Gyrotheodolite measures azimuth from shaft to tunnel.
Total stations extend the control network underground.
Challenges:
Requires stable mounting (vibration affects gyro accuracy).
Time-consuming compared to GNSS-based methods.
2. Monitoring Tunnel Boring Machines (TBM)
Ensuring TBMs stay on course requires real-time surveying and guidance systems.
Key Monitoring Parameters:
Steering (Pitch, Roll, Yaw): Laser targets/prisms track TBM position.
Advance Rate: Measures progress vs. geological conditions.
Segmental Lining Deformation: Detects stress changes during/after installation.
Technologies Used:
Automated Total Stations (ATS): Continuously monitors prism arrays on the TBM.
Inertial Measurement Units (IMUs): Measures TBM orientation in real time.
Laser Guidance Systems: Projects alignment lasers for manual checks.
Case Study:
Issue: A TBM deviated 150 mm due to soft ground.
Solution: Adjusted cutter head pressure based on survey data.
3. Subsidence and Settlement Analysis
Tunneling-induced ground movement must be monitored to prevent surface damage.
Causes of Subsidence:
Ground Loss: Soil displacement during TBM excavation.
Water Table Changes: Dewatering for construction can cause sinking.
Structural Loads: Buildings above tunnels may settle unevenly.
Monitoring Methods:
Prism Networks: Surface targets tracked by robotic total stations.
Inclinometers: Measures lateral soil movement near tunnel walls.
Fiber Optic Sensing: Detects micro-strains in real time.
Data Interpretation:
Settlement Troughs: Gaussian distribution models predict affected zones.
Thresholds: Typically 10–30 mm allowable settlement for urban areas.
Mitigation Strategies:
Compensation Grouting: Injects material to stabilize sinking ground.
TBM Parameter Adjustments: Reduces ground disturbance.
Summary
This module covers advanced underground surveying techniques—from gyroscopic alignment to TBM guidance and subsidence management. Mastery ensures safe, accurate tunneling in complex environments.
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